The described technology relates to the evaluation of structural defects in metals, and in particular, to the evaluation of structural defects in IN718 alloy forgings.
Nickel alloys demonstrate excellent mechanical properties at high temperatures. As a result, these alloys are often used to make components that will be subjected to high loads and high temperatures in service. IN718, for example, is one particular nickel alloy that is frequently used to make internal components for large land-based gas turbines. Although nickel alloys do demonstrate excellent mechanical properties, defects can occur in nickel alloy forgings during the melting and forging processes due to the chemistry of the alloys. Not all of these defects are unacceptable, however, and conventional fracture mechanics analyses are frequently employed to show that many of these defects can be accepted.
Evaluating a forging defect can prove to be a complicated and time-consuming process using conventional analytical methods. These methods often require considerably more information about the defect than just the basic parameters of defect size, defect location, and defect operating stress and temperature. For example, conventional methods for evaluating defects in IN718 forgings can additionally require other parameters known to those of skill in the fracture mechanics art, such as the offset of the defect from the forging surface, the fracture toughness of the material, the threshold of fracture toughness of the material, and the surface crack distortion factor. Perhaps even more onerous, these methods often require solving esoteric fracture mechanics equations such as the Paris equation for crack growth rate (to obtain the coefficient C and exponent n), and the Walker equation for the stress intensity factor (to obtain the exponents m+ and mxe2x88x92). As will be appreciated by those of skill in the relevant art, an engineering background in fracture mechanics, if not a basic prerequisite, can greatly facilitate using conventional methods to evaluate forging defects.
FIG. 1 is a flow diagram illustrating how a typical forging evaluation using conventional methods might proceed after ultrasonic testing has indicated the presence of a defect in the forging. In block 102, a non-destructive test (xe2x80x9cNDTxe2x80x9d) engineer determines the size and location of the defect and provides this information to a design engineer. The design engineer in block 104 determines the operating stress and temperature at the location on the component to be made from the forging that corresponds to the location of the defect on the forging. The size, location, stress, and temperature information is then provided to a fracture mechanics (xe2x80x9cFMxe2x80x9d) engineer who in block 106 uses this information, along with additional fracture mechanics parameters such as those discussed above, to determine the number of life cycles the finished component would safely survive if made from the forging in question. The resulting number of life cycles is then provided to the design engineer in block 108. One xe2x80x9clife cyclexe2x80x9d in this context refers to one on/off cycle of the machine in which the finished component is ultimately installed. For example, if the finished component is ultimately installed in a turbine, then one life cycle is equivalent to turning the turbine on and off one time.
In decision block 110, if the design engineer determines that the finished component will survive a sufficient number of life cycles (5000 in this example), then the forging is accepted. If the design engineer determines that the component will not survive a sufficient number of cycles, then in block 112 the design engineer will try to reorient the finished component with respect to the uncut forging in an effort to relocate the defect to a region of lower stress or temperature. If the design engineer is successful in relocating the defect, then the new lower stress and temperature parameters are provided back to the fracture mechanics engineer. In block 114, the fracture mechanics engineer reevaluates the component using the new lower parameters, and provides the new life cycles estimate back to the design engineer in block 116. In decision block 118, if the life cycles requirement is now satisfied by the new life cycles estimate, then the forging is accepted. Otherwise, the forging may be finally rejected.
As the foregoing example illustrates, it can be a lengthy process from the time a defect is initially detected to the time a final decision regarding forging acceptance is made. In addition, the process can require several iterations and the participation of several different specialists, most notably, a fracture mechanics engineer. Accordingly, an accurate and user-friendly method for quickly and simply evaluating forging defects that does not require an expertise in fracture mechanics would be desirable.